Crutches: Weight Distribution And Weight Transfer

1/11The correct weight distribution is a product of the setup in the car as well as the type of racetrack on which you are running. The weight distribution we need tells us a lot about how good our setup is.

Weight distribution and its relationship to our setup is more complicated than many choose to believe. We have learned how weight distribution affects our setups and how we can improve our setups to take advantage of different weight distribution ranges related to specific types of race cars and various racetracks.

Weight distribution can be defined in several ways. It can mean the placement throughout the car of the physical weight and can be read in percentages, such as left-to-right-side percentage, front-to-rear percentage, and sprung and unsprung weight. It can also be read as center of gravity height. These numbers are fixed for a particular race car until we move weight around in the car. We need to know the dynamic weight distribution, which means reading the weight of the car with the driver and all fluids included just as it would be on the racetrack.

The other measure of weight is related to how much each tire supports the total dynamic weight of the car in percentages. The amount of weight that is supported by each tire can and does change when we physically adjust the heights of the springs and as the car travels around the racetrack.

2/11Coilover spring/shock combinations have adjuster rings on the threaded portion of the shock to set the spring height and regulate the amount of weight that is supported by each tire.

Weight jacking bolts on big-spring cars and the adjuster rings on coilover shock/spring types of cars are the tools used to change the static distribution of weight. This changeable distribution of weight is what we often use to tune the handling balance of our race cars.

Weight distribution in our cars affects the handling balance. It is an adjustment tool for both finding the best handling balance and tuning the setup to a driver's preference. Some drivers like a car that is somewhat freer while some cannot drive a loose car. The reading of the distribution of the total vehicle weight on the four tires is known to pavement racers as crossweight percentage or wedge, and left-rear weight to dirt racers. Both tell us basically the same thing.

As the car begins to push, the driver turns the steering wheel more until the front has developed more traction than the rear and the car starts to go loose. This usually happens just as we are applying the throttle to get off the corner and the car goes very loose-quickly. The tight condition has led to a loose-off condition that may heat up the right-rear tire and cause it to wear excessively and lose grip. We are now abusing two tires because the car has a tight setup.

3/11The cars that use stock springs have weight jacking screws at the front and rear that are mounted with a housing welded to the frame to adjust the height of the top of the spring. The top of the screw has a square hole to fit a 1/2-inch drive extension. A mark should be painted on one side of the screw so the number of turns can be easily counted.

We can tell a lot about how a race car is set up and how balanced it is by analyzing the crossweight percentage that will work with the setup to make the car neutral in the turns. We must remember that a neutral-handling car is not necessarily a well-balanced car dynamically from the perspective of how the front and rear are working together.

Here, we will use the term crossweight percentage to represent the distribution of weight among the four tires. We find the crossweight percentage by adding the weight on the right-front and left-rear tires together and dividing that number by the total vehicle weight. The most important thing to know about crossweight is that we cannot build the setup around a particular number, but rather tune the crossweight to a balanced setup.

If the setup is truly balanced, then the neutral handling that we tune to with crossweight will be consistent and stay that way for many laps. If the car is unbalanced in the setup, the neutral handling will go away in a short time, usually from 10 to 25 laps.

4/11This new coil spring adjuster system from AFCO allows the spring top to conform to the compression of the spring as the car negotiates the turns, providing more consistency.

Unbalanced setups go away because one tire is doing too much work. It used to be the right-front (RF) tire, but with the super soft setups we see on asphalt race cars, the right rear (RR) could be the one that is being abused.

If the setup is unbalanced to a great degree, with the RF tire working too hard, we may end up with a tight/loose condition. This is where, as we have discussed in past articles, the rear suspension is wanting to roll more so than the front suspension. This condition causes excess weight to transfer to the RF tire, and the front pair of tires becomes less equally loaded. Less equal loading causes less overall traction from a pair of tires, and a push develops.

To compensate for the push, we turn the steering wheel more to the left and the front tires gain more of an angle of attack. In past articles, we learned that the greater the angle of attack a tire has, the more traction it develops-to a point. The bottom line is that the RF tire is working very hard to turn the car and it will heat up and wear excessively. In a short time it will lose a lot of its ability to grip the racetrack, and the car will start to push.

5/11A spring rubber changes the rate of the spring as the spring compresses and decompresses. This changes the weight distribution among the four tires. Because this is a variable, depending on the amount of spring compression, it is not an advisable method for increasing spring rate.

This car started out neutral but became very unbalanced. That is exactly why unbalanced setups do not win most races. To be perfectly honest here, a short race of 25 or 30 laps can be won by an unbalanced setup. These short runs do not provide enough time for the car to abuse the tires to the point that it slows down sufficiently to lose the lead. That being said, the balanced setup will still be as fast or faster than one that is unbalanced.

Another aspect of crossweight is that for every race car and setup there are several ranges of crossweight that will cause the handling to be neutral. This is not universally known, but through literally hundreds of cases, it has been proven that one car may have as much as three ranges of crossweight that will make the car neutral.

For most short track cars, given a particular vehicle weight distribution, there is a low and a high range of crossweight that will make the car neutral. With the same springs, moment center locations front and rear, and so on, the car will be neutral at say 51.2 percent and at 58.6 percent. In-between is "no man's land." The truth is that we can find a certain percentage of crossweight in each range that will make the car neutral in handling.

6/11A tool that facilitates changing springs and doesn't alter the weight distribution is this single-wheel scale by Intercomp. The weight is read with the old spring. Then, with the new spring installed, the jack screw is adjusted until the same weight is read.

If there are at least two ranges to choose between, how do we know which to use? What we have found is that the lower crossweight range works best on the higher-banked tracks of 12 degrees or more. For the lower-banked tracks, the high range works better to provide better traction off the corners under acceleration by increasing the loading on the LR tire. On the high-banked tracks, the banking usually creates enough mechanical downforce to provide sufficient traction so the tires will not lose grip under acceleration.

The very best dynamic weight distribution (while in the turns) occurs with two sets of equally loaded tires at midturn. In the low range, the outside tires will be equally loaded in the turns, with the inside tires being equally loaded as well. In the high range, the RF and LR tires will be equally loaded and the LF and RR tires will be equal. In this way, at either range we have more equally loaded pairs of tires at the front and rear to provide the most traction available so we can go through the turns as fast as the car is capable of going.

7/11This is how the weight should be distributed at midturn when we use the low range of crossweight. After the weight has transferred as the car negotiates the turns, the same amount of weight is supported by both right-side tires as well as both left-side tires. This is how it should happen with a balanced setup.

The need to find a setup that will provide the best weight distribution at midturn is the reason we seek a balanced setup. The balanced setup causes the weights to transfer predictably at each end of the car. Unbalanced setups are very unpredictable because the weight transfer changes as the tires wear or the driver changes driving lines.

Different track banking or surface changes that alter the track's grip characteristics can cause a handling balance change with an unbalanced chassis. Handling balance may also change as the track surface temperatures increase or decrease. From afternoon (warm) to night (cooler), the handling will change with an unbalanced setup, whereas a balanced one stays more consistent.

We can predict the best crossweight for a car based on its weight layout, or where the weight is placed in the car. The front-to-rear distribution of weight dictates the crossweight percentage to use. As rear percentage increases, more crossweight is necessary in order to be neutral for each range.

8/11This is an example of how the weight would be distributed at midturn when we use the high range of crossweight. After the weight has transferred as the car negotiates the turns, the same amount of weight is supported by the RF and LR tires and the LF and RR tires would be supporting the same weight. The setup must still be balanced to achieve this result.

If we know a car needs 52.2 percent crossweight to be neutral based on the front-to-rear percentage, then running 49 or 50 percent in a neutral car means the setup is unbalanced. The RF tire is working too hard and the car is tight because reducing the crossweight makes the car more neutral. This change does not solve the problem with the unbalanced setup, and the RF tire is still doing too much work. What is worse is that the car is probably loose off the corner and the crew makes changes that will tighten it, making the original problem worse.

Another interesting phenomenon is that as the applied g-forces change, so does the crossweight requirement for a particular car. The higher the g-forces generated, the more crossweight percentage needed. If our car's lap times tend to fall off a lot over a fairly short run, the g-forces are dropping off also (less turn speed means less g-force). So the crossweight we have chosen will not be sufficient as the car slows.

To stay neutral in handling, the required crossweight should come down as well. Since it cannot change, the car would become tighter by having too much crossweight percentage as the lap times continue to fall off. One advantage of a balanced setup is that the lap times stay more consistent, hence less change in the g-forces, as well as the required crossweight. The car remains more neutral as well.

9/11The term balanced setup refers to a combination of spring rates, moment center locations, and other setup parameters that will result in both ends of the car wanting to do the same thing in the turns. That is defined as desiring to roll to the same angle. Once we can match up the two ends, the weight transfer is then predictable and consistent.

1. Lap times fall off considerably after approximately 20 laps while the leaders stay consistent.2. The RF or RR tire shows excessive heat and wear compared to the other tires.3. The LF or LR tire shows considerably less heat and wear than the other tires.4. The RR tire shows extreme heat and wear from spinning off the turns5. The driver uses excess steering input at midturn compared to other faster cars.6. The car "snaps" loose just when the driver gets into the throttle.

As you make changes to cause the car to be more balanced, the crossweight percentage needs to increase to keep up with the changes. If you raise the rear Panhard bar or stiffen the right-rear spring to reduce the rear roll angle to match the front, the car will become loose if you do not also increase the crossweight.

10/11When we change the front-to-rear percentage of total weight in the car, the crossweight needs to change to keep the car neutral in handling. This is a 2,800-pound Late Model with a 66-inch track width, a 56 percent left-side weight, and a 15-inch center of gravity running on a 12-degree banked track pulling 1.7 g's in the turns. The numbers in the chart were calculated using standard weight transfer dynamic formulas available in many books on the subject. The car is considered to have a balanced setup.

The process of balancing the car makes the LF tire work harder for conventional setups on dirt and asphalt, and it makes the LR tire work harder for the BBSS setups. Too many teams make adjustments that make the car more balanced only to back off because the car is getting too loose or too tight.

Weight Do's:1. Weigh your car with the driver to know your exact weight distribution.2. Level your scales, roll the car to take out all binding in the control arms, air up the tires to race pressures, and add all fluids before measuring the weights.3. Make changes to the crossweight in the shop to know how a turn of the screw or ring affects the crossweight percentage.4. Change all four corners when making changes to the crossweight percentage to maintain the correct ride heights. To put cross in, put turns into the RF and LR springs and take turns out of the LF and RR springs. The opposite takes cross out.5. Make records of your weights. Use weights taken on the racetrack scales for reference only. Do not ever change your crossweight to match the track scales. They are almost never level.

11/11Coilover springs are much easier to adjust when changing the weight distribution. Make sure there are no binds in the mounting points that will affect your readings. This mounting bolt needs a tapered spacer between it and the shock eye.

Some Weight Don'ts:1. Don't build your setup around a particular crossweight percentage just because you have always run that number. Be open-minded in order to find a better combination if that is what you need.2. Don't guess at the crossweight percentage in your car. The old way of jacking up the rear end with a large socket between the jack and the rear differential pumpkin and seeing how far off the ground the RR tire is when the LR tire is barely touching is way out of date.3. Don't run the high crossweight range on high-banked tracks unless the banking falls off abruptly and there is a problem getting traction off the corners. And don't try to run the low range on the flatter tracks with bite-off problems like I did at one time. That is how I learned about the different ranges.4. Don't move weight around in the car to effect changes to the crossweight percentage. Move the weight jacking devices or shim the springs in a stocker.5. Don't try to match your crossweight to a competitor that is running well. His overall weight distribution and setup may be very different from yours, especially the front-to-rear percentage of weight. This setup stuff all works in combination, and each car's combination is necessarily different.

Weight data is a part of that bundle of information about your car that you need in order to be competitive. Don't be shy about experimenting with items such as weight distribution, and by all means, think outside the box.

Once you find that perfect combination that the car needs, you won't share it, just like no one shared with you. Then you'll know why it takes so long to be competitive. It takes getting to the point where you quit listening to your competitors and start thinking for yourself, no matter how long you've been racing.